Generally, a polymer material may be described by the following three quantities: number-average mass, denoted below by Mn; weight-average mass, denoted below by Mw; and the polydispersity index PI.
These three quantities are sufficient to describe a homopolymer or a copolymer, provided that its chemical composition is well known. In fact, the following definitions will demonstrate that a gradient copolymer has to be described much more specifically and that the parameters of its synthesis will be determining in order to describe it correctly.
In what follows, the abbreviation Tg denotes the glass transition temperature of a polymer.
In the present invention, the term “hydrophilic monomer” will denote, without distinction, monomers having corresponding homopolymers which are soluble in water or dispersible in water or having an ionic form which is soluble in water or dispersible in water.
A homopolymer is said to be soluble in water if it forms a clear solution when it is in solution at 5% by weight in water at 25° C.
A homopolymer is said to be dispersible in water if, at 5% by weight in water at 25° C., it forms a stable suspension of fine, generally spherical, particles. The mean size of the particles constituting said dispersion is less than 1 μm and more generally varies between 5 and 400 nm, preferably from 10 to 250 nm. These particle sizes are measured by light scattering.
The following definitions are of use in making it possible to distinguish gradient copolymers from other copolymers and polymers:
Controlled Radical Polymerization:
                Radical polymerization controlled by nitroxides. WO 96/24620 or WO 00/71501 discloses the devices of this polymerization and their use. The scientific understanding of such a control technique is described by Fischer in Chemical Reviews, 2001, 101, 3581, by Tordo and Gnanou in J. Am. Chem. Soc., 2000, 122, 5929, and Hawker in J. Am. Chem. Soc., 1999, 121, 3904, for example.        Atom transfer radical polymerization. Disclosed in WO 96/30421, it proceeds by reversible insertion with regard to an organometallic complex in a bond of carbon-halogen type.        Radical polymerization controlled by sulfur derivatives of xanthate, dithioester, trithiocarbonate or trithiocarbamate type. Reference may be made to the following documents: FR 01/02848, WO 02/068550, WO 98/01478, WO 99/35177, WO 98/58974, WO 99/31144 or WO 97/01478, a reference publication in the field being: Rizzardo et al., Macromolecules, 1998, 31, 5559.        
Controlled radical polymerization denotes polymerizations for which the side reactions which usually result in the disappearance of the propagating entities (termination or transfer reaction) are rendered highly improbable with respect to the propagation reaction by virtue of an agent for controlling the free radicals. The shortcoming of this method of polymerization lies in the fact that, when the concentrations of free radicals become high with respect to the concentration of monomer, the side reactions again become determining and tend to broaden the distribution of the masses.
Gradient
Gradient copolymer: the gradient copolymer is a copolymer of at least two monomers generally obtained by living or pseudoliving polymerization. By virtue of these methods of polymerization, the polymer chains grow simultaneously and thus incorporate, at each instant, the same ratios of comonomers. The distribution of the comonomers in the polymer chains thus depends on the change, during the synthesis, in the relative concentrations of the comonomers. Reference will be made to the following publications for a theoretical description of gradient copolymers: T. Pakula et al., Macromol. Theory Simul., 5, 987-1006 (1996); A. Aksimetiev et al., J. of Chem. Physics, 111, No. 5; M. Janco, J. Polym. Sci., Part A: Polym. Chem. (2000), 38(15), 2767-2778; M. Zaremski et al., Macromolecules (2000), 33(12), 4365-4372; K. Matyjaszewski et al., J. Phys. Org. Chem. (2000), 13(12), 775-786; Gray Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) (2001), 42(2), 337-338; K. Matyjaszewski, Chem. Rev. (Washington, D.C.) (2001), 101(9), 2921-2990.Natural gradient: will be used for a gradient copolymer synthesized under batchwise conditions from a starting mixture of comonomers. The distribution of the various monomers in the chain thus follows a law deduced from the relative reactivity and from the starting concentrations of monomers. These polymers constitute the simplest class of gradient copolymers as it is the starting mixture which defines the final product property.Artificial gradient: will be used for a copolymer, the concentration of monomers of which will be varied during the synthesis by a processing stratagem.Composition gradient: this is the G function defined by
            G      ⇀        ⁡          (      x      )        =      ∑                  ⁢                            [                      M            i                    ]                ⁢                  (          x          )                    ⇀      where x denotes the standardized position on the polymer chain and [Mi](x) denotes the relative concentration in this position of the monomer Mi (expressed in moles). In the case of isoreactive monomers, [Mi](x)=½.
The G(x) function thus describes the composition of the gradient polymer locally. Two copolymers can have an equivalent overall composition but very different G(x) functions. The factors determining the G(x) functions are the relative reactivity coefficients of the monomers (ri for the monomer Mi) (which depend mainly on the type of process of synthesis (homogeneous, disperse) and on the solvents, and the like), the starting concentrations of the monomers and the additions of monomers during the polymerization. Mention may be made, by way of example, of the textbook case of a gradient copolymer of styrene (M1) and of methacrylic acid (M2) in a homogeneous polymerization system. The literature gives us r1=0.418 and r2=0.6.
The variation in the starting concentrations of styrene and of methacrylic acid makes it possible to obtain different gradient copolymers thus having chains with completely different structures. Thus, at 10% of methacrylic acid, a very low gradient copolymer is obtained for which nanostructuring cannot be expected, at 20% a copolymer with a hydrophilic “head” and a hydrophobic tail is obtained with a sufficiently pronounced gradient to result in nanostructuring. On the other hand, at 50%, the monomers being isoreactive under these conditions, the copolymer obtained is of the alternating type.
Despite the fact that each of the polymers described is a gradient polymer of styrene and of methacrylic acid, the difference in starting concentration of the monomers results in chains with completely different structures, conferring different properties on the copolymers.
This example thus illustrates the importance of the starting monomer concentrations on the arrangement of the different monomers along the chain.
Block Copolymer:
This is in fact a subclass of the family of the artificial gradient copolymers. [Mi](x) is then the product of at least one Dirac function by a monotone function, which reflects the fact that the mixture of monomers changes from one form to another in the chain due to a change undergone by the monomers in the reaction medium (stripping of the first mixture or addition of at least one new monomer). Some block copolymers are distinguished in that they have a random hinge between the two blocks.
In WO 02/068550, the authors disclose and claim block polymers of (AB)n-core type (with n greater than or equal to 2) with optionally a hinge between the A and B blocks; such structures correspond to the following GA(x) curves, as shown in FIG. 1:
It is therefore seen that the structure of such a chain is:                symmetrical        the notion of block is reflected by a zone at least of the chain where the composition of A is 1 and of B is 0,        such structures correspond to a process of synthesis based on the polymerization of a first mixture of monomers and then of a second with optionally purification between the two stages of the polymer to avoid the hinge block.        
Such compounds are nanostructures and therefore exhibit physical and rheological properties in keeping with this nanostructuring: the order/disorder temperature is high. Up to this temperature, the viscosity of the polymer is very high and one of the consequences is that it is difficult, indeed even impossible, to dissolve such polymers in aqueous solution or in a solvent.
It emerges from these definitions and from these examples that the G(x) functions, the number-average mass and the polydispersity index are three necessary and sufficient pieces of information which differentiate one gradient copolymer from another. It also emerges that the G(x) function is directly determined by the process of synthesis.
Surprisingly, the Applicant Company has discovered that amphiphilic gradient polymers having at least one hydrophilic monomer and at least one monomer M1, the homopolymer of which corresponding to a Tg1 of less than 20° C., exhibit the advantage of being easy to handle in water or in a solvent in comparison with block copolymers while retaining rheological properties which are advantageous for applications such as paint, varnishes, adhesion, filler dispersion or even formulations for medical, dermatological or cosmetic use, it being possible for the monomer M1 to be a hydrophilic monomer.